The presence of sulfur compounds in hydrocarbon streams is undesirable since those compounds may entail various problems associated with the transportation, storage or practical use thereof. Sulfur compounds are corrosive to the piping used to carry those fluids, as well as to the tanks and vessels wherein they are stored. When the hydrocarbon streams are used as intermediates in chemical processes they may cause deactivation of the catalysts used in such processes. When the hydrocarbon streams are used as fuels, the sulfur compounds react to produce sulfur oxides, and eventually other sulfur compounds, which remain present in the combustion gasses that are released into the atmosphere. Those compounds are subject to severe restrictions regarding their release into the environment. Such motives justify the treatment of liquid and gaseous hydrocarbon streams for removal of sulfur compounds contained therein down to concentration levels that might be deemed acceptable.
It is well known that certain products formulated with metal oxides and hydroxides may be used to remove sulfur compounds from liquid or gaseous streams. The cited metal oxides have the generic formula MxOy, and the metal hydroxides have the generic formula MxOy(OH)z, wherein M represents a metal. The main constituent metals of those oxides are zinc and iron or mixtures thereof. Other metals such as copper, nickel, cobalt, molybdenum and manganese may also be present, however their main function is to increment the rate of absorption of impurities by the main metals. The end products based on such oxides and hydroxides may be presented as powders, that are used in the form of slurries in mixture with the stream to be treated, or in the form of granules that are accommodated on fixed beds through which passes the stream of fluid to be treated. The granules may be supported, when the metal oxides and/or hydroxides are deposited on inert solids, or the granules may be massive when constituted only by the metal oxides and/or hydroxides and their additives.
Upon exhausting their useful capacity, the absorbents used in the form of granules are removed from the equipment units in which they were used, and are substituted with new charges to continue the process. For this reason, there are used multiple absorption equipment units, arranged in parallel, such that it is always possible to interrupt the operation of one of them in order to replace the absorbent without interrupting the fluid treatment operation as a whole.
It will always be desirable that the exhausted absorbent might be recovered and reprocessed in order to avoid the generation of solid residue that might face serious restrictions for disposal into the environment. It is further desirable that the absorbent have a high capacity to absorb impurities in order to extend its practical utilization, both in terms of operating intervals and in relation to weight of absorbent used per unit weight of impurity removed. A higher absorption capacity affords a longer operating time and a lesser number of replacement operations with less incidence of the cost of removal of the same amounts of impurities.
For the treatment of gaseous streams at relatively high temperatures (above 100° C.), one of the most used products is mainly formulated with zinc oxide (ZnO). The elimination of H2S involves the following chemical reaction:ZnO=H2SZns=H2O
In operating conditions, the water that is produced is vaporized, transforming itself into a gas, which is incorporated to the gaseous stream that is being treated.
The zinc oxide based absorbents are almost always massive, where the zinc oxide is used together with a binder to maintain the physical integrity of the final product.
At relatively low temperatures, operating with water-saturated gas streams, the products formulated based on zinc oxide are generally not preferred due to their low absorption rate and the possibility of physical disintegration of the product granules, which may cause excessively increase the pressure drop across the absorbent bed prior to its chemical saturation.
The activated carbon has been used for absorption of H2S from streams of natural gas and other gasses, due to its large specific area. However, it has a low H2S retention capacity, operating at low temperatures. For that motive, the use of activated carbon in applications of that nature has been quite limited. The use of activated carbon is more common for removal of undesirable odors (including that of H2S) in gasses that are directly released into the atmosphere.
For operation at relatively low temperatures and with streams saturated with water vapor/steam, the generally preferred absorbents are those based on iron oxide in the form of crystalline magnetite (Fe3O4), that may also contain iron hydroxides (FeO(OH)). Thus, such as in the ease of absorbents based on zinc oxide, the removal of H2S also occurs by way of a chemical reaction, involving the reduction of iron of valence 3 to valence 2:Fe3O4+4 H2S →3 FeS+4 H2O+⅛ S8 2 FeO(OH)+3H2S→2 FeS+4 H2O+⅛ S8 
It is also possible, as well as desirable, that a reducing dissolution occur, such as shown in the equation below:Fe3O4+6 H2S→3 FeS2+4 H2O+2 H2 
The hydrogen may participate in the reaction of absorption of the H2S with the formation of FeS:Fe3O4+3 H2S+H2→3 FeS+4 H2O
Some additives may accelerate the rate of absorption of impurities contained in the streams treated with absorbents based on iron oxide and hydroxide. Examples of such additives include copper oxides (cupric or cuprous copper), as proposed by Dalbert Scranton in patent application No. WO 98/07501, published in Feb. 26, 1998.
The first absorbents based on iron oxides were prepared using inert solid supports whereon the oxides were deposited. One of the inert solid supports was made of wood pieces impregnated with iron oxide. Absorbents of that type were in commercial use for a long time, however they entail serious disadvantages. Some of the disadvantages consist in a low capacity to absorb impurities, per unit volume of absorbent, and the tendency to excessive packing/caking of the bed, which may be caused by retention of water, leading to an increasing head loss to very high levels that require the substitution thereof. One other problem resides in the difficulty to recover and recycle the exhausted product in a practical and cost-effective manner.
Various materials were proposed as substitutes for the wood as support for the metal oxides and hydroxides. One of these is amorphous iron (III) oxide (Fe2O3), as proposed, for example, by Irwin Fox et al in U.S. Pat. No. 4,366,131. In turn, Jerome Gross, in patent application No. WO 91/03422, page 4, proposes the use of calcined montmorillonite as support for iron oxide based absorbents. A formulation that is preferred by Jerome Gross indicates a proportion of 59% montmorillonite, 22% iron oxide, 18% water and 1% sodium sulfite. In one of the tests having been conducted, that product evidenced a content of 288 kg/m3 of iron oxide per m3 of bed (18 lbs. of iron oxide per cubic foot of bed).
The supported absorbents exhibit an inherent deficiency consisting in the presence of inert material (support), when it is desired to maximize the amount of reactive iron oxide and hydroxide per unit volume. The inert material takes space without contributing to retain the impurities from the treated stream. On the other hand, massive absorbents may allow the accommodation of much higher amounts of reactive iron oxides and/or hydroxides per unit volume Extruded massive absorbents based on iron oxides useful for the removal of sulfur compounds from process streams were already indicated for commercial use in the book “Catalyst Handbook”, edited by Wolfe Scientific Books, London, 1970.
One of the main difficulties entailed by the use of massive absorbents of iron oxide, particularly of magnetite, resides in the difficulty in maintaining the integrity of the shape of the final product during the preparation thereof and handling prior to its use and particularly upon the same being exhausted, when the magnetite transforms into the iron sulfides. Several factors are associated with the maintenance of integrity of the particles during use and unloading. The designed material may not stay aggregated in the presence of water, which is commonly entrained in some gaseous streams, and which may cause compacting of the bed. Furthermore, the higher is the conversion of the oxides to sulfides, the higher will be the probability of disintegration of the granules. The disintegration of the granules during the use of the product may lead to the formation of fine particles that lodge in the void spaces in the absorbent bed, thereby increasing resistance to passage of the fluid (increased head loss), where it may be necessary to stop and substitute the absorbent even before the same reaches chemical saturation. Thus, even with high contents of metal oxides, there might not be achieved a higher capacity of removal of sulfur compounds. The disintegration of the material may also entail the release of powder during the handling and unloading of the exhausted charge, which constitutes a serious disadvantage from the perspective of industrial hygiene. In addition, the formation of powder represents a loss of product to the environment, which might represent a serious pollution problem.
The preparation of granulated materials by compaction or extrusion almost always requires the use of at least one binder capable of keeping together the particles of the metal oxides and hydroxides. The binders may be inorganic compounds or organic compounds. Examples of inorganic binders include bentonite, kaolin, cement and alumina. Materials of this type are described, for example, by Gyanesh P. Khare, in patent U.S. Pat. No. 5,306,685. Mr. Khare used those materials to prepare extruded absorbents constituted by mixtures of zinc oxide and iron oxide (Fe2O3). Examples of organic binders may include starch paste, sugar (sucrose), glucose, gelatin and others, cited generically in PERRY's Chem. Eng. Handbook, 6th Edition (1984). Polyethylene glycol (PEG) and poly (vinyl alcohol) are cited by Koichi Kitahara et as in U.S. Pat. No. 5,670,445. In addition to these authors, Mahesh C. ha et al, in U.S. Pat. No. 4,732,888, indicate the use of starch, in addition to methylcellulose and corn syrup, for producing absorbent tablets containing iron and zinc oxides.
For extruded absorbents, it is necessary to prepare a paste having sufficient plasticity to allow the same to flow through an extruder, The addition of water in adequate proportions may produce a paste with the desired properties for extrusion. An excessive amount of water may produce an extrudate with mechanical strength that is too low for handling during the subsequent operations (cutting, transport and drying), A low amount of water may leave the paste without fluidity and very abrasive to the point of rendering the extrusion operation unfeasible. The addition of water to obtain a mass of iron oxide with adequate properties for extrusion is described by Paul R. Pine et al, in U.S. Pat. No. 2,457,719, and in U.S. Pat. No. 2,461,147, E. P. Daves et al also describe the addition of water to iron oxides for obtaining pastes with sufficient plasticity for extrusion. These authors also indicate the use of other extrusion adjuvants, such as stearic acid, hydrogenated vegetable oil and tannic acid.